The development of technology for wireless power transfer systems has become widespread in recent years. Japanese Patent Laid-Open No. 2012-139010 discloses a technique for transferring power with high efficiency through impedance matching between a power receiving antenna and a power generating unit that generates DC power.
A case such as that shown in FIGS. 1A and 1B, where power is transmitted from a single power transmitting apparatus to a plurality of power receiving apparatuses, can be considered as an example of the actual operation of a wireless power transfer system. FIG. 10 is a block diagram illustrating an example of the internal configuration of a typical power transmitting apparatus. In FIG. 10, 1000 indicates a constant voltage source that serves as a power source for a class E amp 1001. 1002 indicates a choke coil that prevents power converted to AC by the class E amp 1001 from returning to the DC constant voltage source 1000, whereas 1003 and 1004 indicate resonant capacitors that resonate with a resonant coil 1005. 1006 and 1007 indicate matching elements for a power transmission antenna coil 1008. 1009 indicates a control unit, such as a CPU, that has a function for controlling the constant voltage source, an oscillator 1010 of the class E amp, and so on. In this type of circuit, the CPU adjusts the voltage of the constant voltage source 1000 so that a current required by the class E amp can be supplied from at least one of the outputs of a voltage detection function and a current detection function (not shown) provided in the constant voltage source.
Next, a case where a state has changed from that shown in FIG. 1A, in which a power transmitting apparatus 100 is transmitting power to two power receiving apparatuses 101 and 102, to that shown in FIG. 1B, where the power receiving apparatus 102 has been removed, will be considered. FIG. 11 shows an example of variation in an output voltage of the constant voltage source 1000 and an AC voltage in the power transmission antenna coil in the power transmitting apparatus 100, and variation in an AC voltage of a power receiving antenna coil in the power receiving apparatus 101 that has not been removed, that occur at this time. In FIG. 11, a dotted line indicates a DC output voltage of the constant voltage source 1000 in the power transmitting apparatus 100, a thin solid line indicates the AC voltage at the power transmission antenna coil, and a bold solid line indicates the AC voltage at the power receiving antenna coil of the power receiving apparatus 101 that has not been removed. A state (1) indicates a period in which the two power receiving apparatuses 101 and 102 are receiving power, and a time t0 indicates a time at which the power receiving apparatus 102 is removed. A state (3) indicates a period in which power is being supplied in a stable manner to the power receiving apparatus 101 after the power receiving apparatus 102 has been removed, and a state (2) indicates a period of transition from state (1) to state (3).
While power is being transmitted to the two power receiving apparatuses 101 and 102, the power that was to be supplied to the removed power receiving apparatus 102 becomes a surplus immediately after the time t0 at which the power receiving apparatus 102 is removed, resulting in a state of overvoltage in the power transmission antenna coil and the class E amp of the power transmitting apparatus 100. Because the power transmission current drops due to the power transmitted to the removed power receiving apparatus 102 and the resulting surplus power, the CPU reduces the voltage of the constant voltage source 1000 (a time t1). Thereafter, the CPU adjusts the voltage of the constant voltage source 1000 in accordance with a current value required for transmitting power to the power receiving apparatus 101 that has not been removed (a time t2).
At this time, the AC voltage at the power transmission antenna coil rises as indicated by the thin solid line due to the overvoltage, then begins to drop as the output of the constant voltage source 1000 drops, and is adjusted to the voltage indicated in the stable state (3). Because the power receiving antenna coil of the power receiving apparatus 101 that has not been removed is in a one-to-one relationship with the power transmission antenna coil of the power transmitting apparatus immediately after the power receiving apparatus 102 is removed and thus couples at a mutual inductance m, the voltage at the power reception antenna coil of the power receiving apparatus 101 at this time enters a state of overvoltage. The voltage occurring in the overvoltage state after the power receiving apparatus 102 has been removed is particularly high in the case where the power receiving apparatus 102 that is removed has been receiving a large amount of power and the power receiving apparatus 101 that is not removed has been receiving a small amount of power. In this case, the power receiving antenna coil, a matching element, a rectifier circuit, and so on in the power receiving apparatus 101 that has not been removed, and a constant voltage source connected to the rectifier circuit, may be damaged due to the overvoltage. In addition to cases where power is being transmitted to a plurality of power receiving apparatuses and a power receiving apparatus that is receiving power is removed, the amount of power transmitted from the power transmitting apparatus can also vary drastically due to a driving apparatus such as a motor that is carrying out positional control being switched from a driving state to a stopped state and so on. Accordingly, it has been possible for other power receiving apparatuses to be damaged due to overvoltage in cases where power is being supplied to other apparatuses as well.
Although Japanese Patent Laid-Open No. 2012-139010 attempts to increase the efficiency of wireless power transfer through impedance matching, it does not take into consideration the possibility that an excessive voltage will be input to the power receiving apparatuses as described above.
Having been achieved in light of the aforementioned problems, the present invention prevents an excessive voltage from being inputted to a power receiving apparatus during wireless power transfer.